The invention is directed generally to control systems for regenerative type dryer apparatus of the type utilized in drying a stream of compressed air from a compressor, prior to feeding a compressed air to downstream utilization equipment.
Regenerative dryers normally include at least two drying towers, in which the compressed air is passed through a drying medium or desiccant to remove moisture therefrom. As the drying medium or desiccant in one tower becomes saturated, it is necessary to take the tower off line and redirect the stream of air to the other drying tower or towers. Alternatively, controls may permit bypassing the towers and feeding air directly to the downstream equipment, in the event it is determined that drying is not required.
Accordingly, most regenerative dryers systems include one or more towers, as well as fairly complex valving and control systems for determining which tower(s) is (are) on line at a given time and which tower(s) is (are) being regenerated. Various control systems have adopted various timing schemes for switching between towers, to allow sufficient time for regeneration of the desiccant or drying medium in each tower, from time to time, and in an effort to assure a continuous flow of dry air to the downstream utilization equipment.
However, the energy requirements for operation under the control of such time-based systems is substantial. For example, heated types of drying systems wherein the desiccant is heated to perhaps 350.degree. F. or higher, require considerable energy in their operation. However, even so-called heatless type dryers have significant energy costs in their operation. That is, these latter dryers may require up to 15% or more of the dried outlet gasses or air stream to purge the saturated desiccant during the regenerating portion of the cycle. Accordingly, the heatless dryers with these purge losses to the system frequently cost as much or even substantially more to operate than the heated dryers.
A third type, so-called heat of compression type dryers, also require heat energy to regenerate the saturated desiccant. However, in most cases the use of the heat of compression dryers is confined to applications where there is already sufficient heat in the compressed air stream and no significant amount of additional heat or regeneration energy is required.
In theory, proper selection of a regenerative dryer for a compressed air or other gas handling system requiring drying of the air or gas stream normally requires a drying system having sufficient capacity to handle a "worst case" situation. This means that the dryer is somewhat oversized for normal operating loads and, it follows, is significantly oversized in the event of reduced drying demands on the system. Accordingly, it will be seen that the energy usage may far exceed the actual drying requirements when such an oversized dryer is selected for use with the typical compressed air or other gas handling system.
Most regenerative systems heretofore in use involve a timing system for alternating between on line and off line time of the two dryers, in some preset cycle. Each time the cycle reverses and one tower begins a regeneration cycle, significant energy is consumed. Moreover, valves, desiccant and other components experience wear and tear in repeated operation. Yet, it will be appreciated from the foregoing discussion that the use of some preset timing cycle will also of necessity involve more frequent switching than necessary to accommodate "worst case" moisture levels. That is, the system will switch somewhat too frequently under normal operation conditions and significantly too frequently in reduced loading situations. Moreover, it has been shown that significant dewpoint or moisture spikes may well occur with such switching systems. In fact, most dryers of this type exhibit elevated dewpoints or moisture content of the outlet air for several minutes following a tower reversal.
Accordingly, it has heretofore been proposed to add dewpoint demand systems to existing preset timing-based cycling systems of the foregoing type. Such a system would ideally delay the reversal or switching between towers while the moisture content or dewpoint of the output stream is within acceptable levels, and permit the reversal or switching between towers when the dewpoint or moisture content exceeds acceptable levels.
However, the dewpoint demand systems heretofore utilized have been custom-designed for use with only a single manufacturer's regenerative dryer system, and perhaps for use with only one model or line in a given manufacturer's line of regenerative dryer systems. Hence, most manufacturers limited their dewpoint demand systems to factory installation as original equipment on their own regenerative dryer systems only. Therefore dewpoint demand controller systems have heretofore been generally unavailable for field installation or retrofitting on existing systems, due to the complexity of coordinating the dewpoint demand system and its components with the existing dryer system control components.
That is, field-installation of a dewpoint demand control system usually required altering the dryer control system significantly, including alteration of the dryer's cycle timer or programmer apparatus. The foregoing considerations were heretofore thought to make in-the-field retrofitting of a dewpoint demand system difficult or impossible. Moreover, it has heretofore been considered that the foregoing considerations made it impossible to offer a dewpoint demand control system which was essentially "universal" in its application, considering the immense variety of regenerative dryer and control systems employed by the many manufacturers offering such systems.